1. Field of the Invention
The present invention relates to a data transmission system as a communication system for a mobile communication and a mobile satellite communication, and a data transmitter and a data receiver which are used in the data transmission system.
2. Discussion of the Prior Art
In a mobile communication, a receiving signal level is greatly varied (in several tens dB or greater) since when propagating, it is subjected to various interferences, e.g., shadowing, fading and the like. One of the measures possibly taken for this problem is to correct the receiving signal level by the utilization of the AGC (automatic gain control). FIG. 8 shows in block form an overall wireless communication system. FIGS. 9 and 10 show also in block form a prior transmitter and a prior receiver in the mobile wireless communication system, which are prescribed in “TIA/EIA/IS-139.1-A” (TIA/EIA INTERIM STANDARD: TDMA Cellular/PCS-Radio Interface-Mobile Station-Base Station-Compatibility-Digital Control Channel) or “RCR STD-32”.
In FIG. 8 showing the communication system, reference numeral 1 is a transmitter for converting a transmission data signal S1 into a radio signal which is to be transmitted in the form of a transmission signal S2; 2 is a channel for transmitting the wireless signal; and 3 is a receiver for extracting reception data S4 from a reception signal S3. In FIG. 9 showing the transmitter, numeral 11 is a burst generator; 12 is a preamble adder; 13 is a modulator; and 14 is an antenna. The preamble adder 12 generates a transmission burst signal S12 by use of the transmission data S1, an information signal S11 output from the preamble adder 12, and the like. A pattern of the preamble is a repetition pattern, e.g., ALL0. The modulator 13 modulates the transmission burst signal S12 and produces a modulated signal S13. The antenna 14 emits a radio wave containing the modulated signal S13. In FIG. 10 showing the receiver, numeral 31 is an antenna; 32 is a low noise amplifier (LNA); 33 is an RF/IF portion for frequency converting a signal S31 output from the low noise amplifier 32; 34 is an AGC portion for automatically controlling a low-frequency signal S32 output from the RF/IF portion 33 so that its output signal level has a fixed value; 35 is a demodulator for demodulating an AGC output signal S33; and 36 is a reception controller for extracting the reception data S4 from a demodulated signal S34 output from the demodulator 35. The wireless communication system thus arranged employs a repetition pattern, e.g., ALL0, for the preamble pattern.
Description will be given about the operation of the thus arranged mobile wireless communication system when it sends a signal from a mobile station MS to a base station BMI. The wireless communication system used here is prescribed in “TIA/EIA/IS-139.1-A”. In the mobile station MS, the burst generator 11 constructs a transmission burst by adding the bits of a ramp (R), sync words (SYNC=synchronization, SYNC+=additional synchronization), and an AGC preamble (PREAM=preamble), to data (DATA, already error-correction coded) to be transmitted. A format of the transmission burst is shown in FIG. 11. In the “TIA/EIA/IS-139.1-A”, the preamble consisting of eight symbols of the π/4 shift modulated as a repetition of “1” and “0” is added as the preamble pattern to the transmission data.
Those symbols of the preamble are phased as shown in FIG. 12. As shown, the preamble portion takes a fixed envelope level. The output signal S12 of the burst generator 11 is modulated by the modulator 13 and amplified, and then the thus processed signal is radiated from the antenna 14. When propagating through the channel 2, the radio signal is greatly affected by fadings (e.g., Rayleigh fading and frequency-selective fading), and its waveform is greatly distorted. Further, the signal level of the radio signal largely varies depending on the distance between the base station BMI and the mobile station MS, shadowing, and fadings. In the base station BMI, the radio signal S3 thus distorted in waveform and varied in amplitude is received by the antenna 31; it is amplified by the LNA 32; and it is converted into a low frequency signal by the RF/IF portion 33. An A/D converter and the like are provided at the input of the demodulator 35. Therefore, the input signal level must fall within a predetermined range of levels. An output signal of the RF/IF portion 33 has a great level variation. The level variation of the output signal must be removed before it is input to the demodulator 35. To remove the level variation, the AGC portion 34 is used. An output signal S34 of the AGC portion 34 is demodulated by the demodulator 35, and applied to the reception controller 36. The reception controller 36 extracts the data portion from the burst signal. The demodulated signal S34 is processed for its error removal, for example, and finally is output as a reception data signal S4.
The operation of the AGC portion 34 for effecting the level correction will be described. FIG. 13 shows a basic arrangement of the conventional AGC portion 34. In the figure, reference numeral 41 is an AGC amplifier; 42 is a level detector; and 43 is a low-pass filter (LPF). A low-frequency signal S32 input to the AGC portion 34 contains a great level variation, as mentioned above. The AGC amplifier 41 of the AGC portion 34 amplifies or attenuate the low-frequency signal S32, and produces an output signal S33. The AGC output signal S33 is input to the level detector 42. The level detector compares a signal level of the AGC output S33 with a reference signal level, and produces a signal representative of a difference between those signal levels. The difference signal is input to the LPF 43. The LPF removes a minute variation of the difference signal and outputs an AGC amplifier control voltage signal or an RSSI (received signal strength indicator) signal S40. The RSSI signal S40 determines a gain of the AGC amplifier 41. The AGC amplifier 41 is thus controlled, and through its control, the output signal strength level of the amplifier 41 is approximate to the reference signal level. The operation of the AGC portion 34 is as described above. A given time is taken till a close loop consisting of the AGC amplifier 41, level detector 42 and LPF 43 settles down in operation. In the “TIA/EIA/IS-139.1-A”, to secure a satisfactory demodulation, as shown in FIGS. 14A and 14B, the AGC output signal (input to the demodulator) is settled down at the preamble (PREAM) portion (located in the head portion of the reception burst format) of the reception burst, so that the AGC output levels of the SYNC and DATA portions (follows the PREMA portion in the reception burst format) are put within a desired range of signal levels.
In a case where the fixed repetition pattern is used for the AGC preamble as mentioned, the radio signal having undergone a frequency selective fading channel improperly operates the AGC portion or circuit. This problem is remarkably revealed in particular in a case where a symbol rate is faster than a fading variation rate and a state of fading little varies within the preamble portion. The frequency selective fading will be described. The frequency selective fading occurs where a delay quantity of a delayed wave, reflected by a distant obstacle, e.g., mountain and building, is not negligible when comparing with the symbol period (FIG. 15).
A fading phenomenon producing a delay quantity of the delayed wave which is at least 1/10 as large as the symbol period sometimes is categorized into the frequency selective fading. If no measure is taken for this fading, the preceding wave interferes with the delayed wave, thereby causing a misjudgment of the received signal (FIG. 16). A composite wave of the preceding wave and the delayed wave is depicted in terms of a frequency spectrum in in FIG. 17. The graph shows a great distortion of a waveform, which is representative of an intensity variation of a reception signal having undergone the frequency selective fading channel with respect to frequency. In this sense, the fading of this type is called the “frequency selective fading”. Thus, the frequency selective fading is more likely to occur with increase of the information rate, or the symbol rate.
In the wireless communication system of the type in which a simple repetition pattern is used for the AGC preamble pattern, when the signal is distorted by frequency selective fading channel, an average reception power of the preamble portion greatly varies depending on a relative phase of the preceding wave to the delayed wave and vice versa. FIG. 18 is a vector diagram showing relative phases of the preceding wave to the delayed wave, and the composite waves of those waves at the relative phases. In the figure, times 1 to 3 indicate consecutive three symbols. In case where the fixed repetition pattern is used for the preamble, a relative phase of the preceding wave to the delayed wave is invariable, and therefore, if the fading rate is sufficiently lower than the data transmission rate, the fixed relative phase will be maintained during the preamble reception. The diagram of FIG. 18 shows three cases: a first case where the preceding wave and the delayed wave cancel out (vector diagrams (1), (2) and (3)); a second case where those waves a little interact with each other (vector diagrams (4), (5) and (6)); and a third case where those waves additively act (vector diagrams (7), (8) and (9)). The relative phase of preceding wave to the delayed wave varies depending on a delay quantity of the delayed wave. Therefore, the relative phase randomly varies with the movement of the mobile station MS. And the average reception power varies with time.
When a random pattern is received (in this case, the data portion will take a substantially random pattern by scramble and the like), the relative phase of the preceding wave to the delayed wave varies also within the burst. Therefore, the preceding wave and the delayed wave additively and subtractively act within the same burst, and the average reception power is uniform over the entire range of the burst, as shown in FIG. 19.
In the wireless communication system where the AGC output signal is settled down at the preamble portion and its intensity level is kept substantially constant in the subsequent data portion, the reception power difference between the preamble portion and the data portion is great, entailing a poor modulation. The operation of the AGC circuit in connection with this is shown in FIGS. 20A and 20B. In this case, viz., the large reception power difference is present between the preceding wave and the subsequent delayed wave (including the SYNC portion), if the AGC output signal is settled down at the preamble portion ((1) in FIGS. 20A and 20B), there is produced an improper output level at the in the data portion. The AGC output signal must be settled down again ((3) in FIGS. 20A and 20B) till the AGC output signal is approximate to the reference value. Thence, the demodulator continues its improper demodulation of the data ((2) in FIGS. 20A and 20B) till the AGC output signal is settled down again.